1. Field of the Invention
The present invention relates to a method for manufacturing a stamper which is an electroformed mold to be used for making an optical information carrying disk such as an optical video disk etc. which bears many minute pits on the major surface thereof.
2. Description of the Prior Art
In a known method, a silver layer is formed through sputtering on a photo resist layer formed on a glass plate as an electro-conductive layer and then a nickel layer is formed on the silver layer through electroforming. Then, the nickel layer as well as the silver plate glass are peeled from the glass plate as the stamper. These steps are illustrated by steps S1 through S7 is a flowchart of FIG. 1.
In the exposing and developing step S1, a flat glass circular plate 1 previously coated with a photoresist layer 2 is exposed to a digitally modulated laser beam in the coated side. The photoresist layer 2 of the glass circular plate 1 is then developed or etched to form pits corresponding to the modulating digital signal carried by the laser beam. The glass circular plate 1 becomes a developed master disk on which many pits (P) are formed at the photoresist layer 2 as shown in FIG. 2A.
In the post-baking step S2, the developed master disk is heated so that the photoresist layer 2 bearing pits is dried and rigidly postcured on the glass circular plate 1 as shown in FIG. 2B.
In the silver sputtering step S3, silver (Ag) is deposited on the photoresist layer 2 bearing pits and the glass circular plate 1 through a sputtering process to form a silver electro-conductive layer 3 on the photoresist layer 2 and the glass circular plate 1 as shown in FIG. 2C. In this way, electro-conductivity is given by means of the silver electro-conductive layer 3 on the surface of the photoresist layer 2.
In the electroforming step S4, nickel (Ni) is continuously and electrically deposited or plated on the silver electro-conductive layer 3 by a predetermined thickness through an electrolytic deposition so to form a nickel electro-conductive layer or stamper 4 as shown in FIG. 2D.
In the nickel stamper separating step S5, the nickel electro-conductive layer or stamper 4 with the photoresist layer 2 and silver electro-conductive layer 3 is separated from the glass circular plate 1 as shown in FIG. 2E.
In the photoresist layer removing step S6, the photoresist layer 2 is removed from the silver electro-conductive layer 3 born on the nickel electro-conductive layer or stamper 4 as shown in FIG. 2F. An alkali solution or organic solvent is used for the removing step.
In a silver electro-conductive layer removing step S7, the silver electro-conductive layer 3 is removed from the nickel electro-conductive layer or stamper 4 as shown in FIG. 2G. In this way, the nickel stamper 4 is produced.
However, there is a problem in the above-mentioned known process that the silver electro-conductive layer 3 is apt to be corroded during the storage period before starting the electroforming step and the electroforming step per se. Therefore, the corroded portion appeared on the silver electro-conductive layer 3 results in damages on some pits of the nickel stamper so as to cause some drop-outs of the recorded digital signal. The corrosion of the silver electro-conductive layer 3, further causes the surface of the silver electro-conductive layer 3 to become rough so that a signal to noise ratio (S/N) is lowered in the reproduced signal, and the reproduced image from an optical video disk is deteriorated.